Demands for higher data rates for mobile services are steadily increasing. At the same time modem wireless communication systems, such as cellular 3rd Generation systems (3G), like the Universal Mobile Telecommunications System (UMTS), and 4th Generation systems (4G), like Long-Term Evolution (LTE), for example, provide enhanced technologies, which enable higher spectral efficiencies and allow for higher data rates and cell capacities.
One example of such enhanced technologies is the use of multiple antennas at base stations, which may also be referred to as NodeB or eNodeB according to 3GPP (3rd Generation Partnership Project) terminology, and/or at mobile terminals, which may also be referred to as User Equipment (UE). In the context of the present specification base stations and mobile terminals will also be generally termed transceiver device. Multiple antennas at the transmitter and/or receiver side may generally be used for spatial information processing comprising spatial information coding such as spatial multiplexing and diversity coding, as well as beamforming In this context Multiple-Input Multiple-Output (MIMO) technology has attracted attention in wireless communications, because it may offer significant increases in data throughput and link range without additional bandwidth or increased transmit power. This is achieved by spreading the same total power over multiple antennas to either achieve an array gain that improves the spectral efficiency and/or to achieve a diversity gain that improves the link reliability. Because of these properties, MIMO is an important part of modem wireless communication standards, such as LTE, for example.
As one form of smart antenna technology active or passive antenna arrays may be used for beamforming to obtain a highly directive antenna beam, which may be used to advantage in order to improve spectral efficiency and/or in order to mitigate interference. This may be achieved by combining a plurality (i.e. ≧2) of closely-spaced and co-polarized antenna elements of the array in a way where signals at particular spatial directions (angles) experience constructive interference and while others experience destructive interference. The combining may be performed by controlling the signal phases of the various antenna elements, wherein the controlling may be done analog in the Radio Frequency (RF) domain or digitally in the digital baseband domain. In other words, the beamforming may be achieved with a so-called phased array, which is an array of antenna elements in which the relative phases of the respective signals feeding the antenna elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. An antenna array may be linear (i.e. 1-dimensional), planar (i.e. 2-dimensional), or even 3-dimensional. In order to achieve correlated signals between adjacent antenna elements their mutual spacing typically is in the range of λ/2 or more, wherein λ denotes the system center wavelength of the wireless communication system.
In order to test the compliance of antenna arrays with given specifications for a wireless communication system, such as UMTS or LTE, for example, there are two known approaches, each having its own disadvantage:
According to existing 3GPP test specifications (see e.g. 3GPP TS 36.141 V10.5.0), defining RF conducted tests, base station receivers or transmitters, or transceivers to be more general, suitable for antenna array operation have to be coupled to a test port via a splitter or a combiner, depending on the RX (receiver) or TX (transmit) case. Conducted tests are performed by direct injection of one or more RF signals into power, interface and communication cables connected to the Device-Under-Test (DUT). Due to the test principle spatial behavior of the air interface experienced with antenna arrays may not be properly emulated.
FIG. 1a illustrates a test setup 100 a conducted receiver (Rx) test. A base station 102 comprising an Rx antenna interface 106 with a plurality of antenna connectors is coupled to a test input port 108 via a splitting network 104. For each Rx test, the test signals applied to the Rx antenna connectors of the antenna interface 106 shall be such that the sum of the powers P, of the signals applied equals the power P, of the test signal(s) specified in the test.
FIG. 1b illustrates a test setup 110 a conducted transmitter (Tx) test. A base station 102 comprising a Tx antenna interface 116 with a plurality of antenna connectors is coupled to a test output port 118 via a combining network 114. For each test, the test signals applied to the Tx antenna connectors of the antenna interface 116 shall be such that the sum of the powers Pi of the signals applied equals the power of the test signal(s) Ps specified in the test. This may be assessed by separately measuring the signals emitted by each antenna connector and summing the results, or by combining the signals and performing a single measurement.
Such RF conducted tests can be carried out with rather low effort. However, if the splitting/combining of the test signals is done with a fixed phase relation between the signals i associated to the single antenna connector (or element), this kind of measurement does not take into account the full spatial behavior of an antenna array, i.e. a radiation or beampattern, which denotes the relative distribution of radiation power as a function of direction (e.g. angle) in space due to beamforming by means of complex beamforming weights, for example. Thus, the compliance with a test specification, e.g. according to 3GPP TS 36.104 V10.5.0, may not fully be proven for spatially aware test scenarios, i.e. scenarios in which wanted and/or unwanted (interfering) signals are meant to be radiated in certain spatial directions. Setting all possible phase relations between the single signals would increase the accuracy but would also increase the test effort significantly.
A method leading to an accurate test of an antenna array and/or the transceiver device coupled thereto which also takes into account the radiation pattern would be the “Over-The-Air” (OTA) test, which takes into account the air-interface. This may be carried out either in an anechoic chamber or in the free field. Here, the electromagnetic field is captured with a probe (test antenna at a certain distance). However, this method leads to significantly higher effort than the aforementioned conducted test with a splitter/combiner. This is a very severe disadvantage especially in performance measurements during mass production of transceiver devices for antenna arrays.
Hence, it is desirable to improve a RF test's meaningfulness and, at the same time, keep the test efforts for mass production or product qualification within reasonable limits